Diffusion vacuum pump system apparatus



Aug. 3, 1965 w. G. BAcHLER DIFFUSION VACUUM PUMP SYSTEMAPPARATUS Filed Nov. 29, 1962 INVENTOR. w. BA c H L E R BY N ATTORNEY 25 tClaims. a. 137--5d5) This invention is related to diffusion pump vacuum systems and more particularly to a device for preventing the migration of the diffusion puimping fluid in such vacuum systems.

In diffusion pump systems, high vacuum technique customarily requires the use of cooling traps positioned between the diffusion pump and the vacuum chamber being evacuated. These traps are expected to maintain on the high vacuum side of the system a diffusion pumping fluid vapor partial pressure corresponding to the pumping fluid vapor pressure at the temperature of the cooling traps cooled surfaces. Such partial pressures will in the case of a water cooled trap amount to only 10" Torr While for a condensed gas (for example, nitrogen) cooled trap the partial pressure will be completely negligible with respect to the total pressure produced by outgassing from the vacuum system walls as well as the back diffusion of hydrogen from the diffusion pump.

However, an operating diffusion pump vacuum system with a pre heat-ed high vacuum chamber will, after a short period of time, demonstrate a slow rise in pressure despite the fact that the cooling traps are functioning perfectly. The rising pressure has been shown to result from an increasing partial pressure of the diffusion pump operating fluid and/or its decomposition products. An exacting study of the construction materials used in such high vacuum system-s shows that the creepage of the pumping fluid in a thin film from the diffusion pump walls along the relatively warm internal walls of the cooling traps into the high vacuum chamber is the cause of the rise in pumping fluid partial pressure.

This creepage of pumping fluid into the high vacuum United States Patent chalmber can be so serious as to cause a pressure rise to i almost the saturation vapor pressure of the pumping fluid at the high vacuum chamber wall temperature. Such an occurrence renders not only the cooling trap itself but also any super cooling of its surfaces practically useless inasmuch as the vacuum chamber walls remain untouched by the cooling. The relatively warm system walls can then cause an order of magnitude increase in the partial pressure of the pumping fluid vapor within the chamber.

Pumping fluid barriers have been utilized in the cooling traps to prevent this undesirable surface movement of the pumping fluid in the diffusion pump systems. Such barriers have comprised metal shields arranged within the cooling traps in such a manner as to require considerable detouring of the migrating pumping fluid over deeply cooled parts, thereby preventing the penetration of the pumping fluid into the high vacuum chamber. However, such arrangements in spite of their high cost and complexity have proven quite unreliable in view of the results desired in high vacuum systems.

It is therefore the object of this invention to prevent in diffusion vacuum pump systems the surface migration of the pumping fluid from the diffusion pump into the high vacuum chamber being evacuated and the subsequent increase in pumping fluid vapor partial pressure resulting therefrom.

One feature of this invention is the provision in a diffusion vacuum pump system of a pumping fluid creepage barrier positioned so as to interrupt all wall surface paths between the diffusion pump and the high vacuum chamber wherein the pumping fluid creepage barrier is com- 3,1932% Patented Aug. 3, 1965 posed of a material which will not be wetted by the diffusion pumping fluid.

Another feature of this invention is the provision of a diffusion vacuum pump system of the above featured type wherein the pumping fluid creepage barrier is composed of a solid material having a surface energy less than that of the diffusion pumping fluid Another feature of this invention is the provision of a diffusion vacuum pump system of the above featured type wherein the pumping fluid creepage barrier is composed of a solid material having a surface energy less than 25 dynes per centimeter.

Another feature of the present invention is the provision of a diffusion vacuum pump system of the above featured type wherein the pumping fluid creepage barrier is composed of a condensed fluorinated hydrocarbon material such as, for example, the materials known by the trade names Teflon, Chemelec-300, Fluoroflex-T, Fluoroplast, Gering TRW, Polypenco-T, and Kel-F.

These and other features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic sectional elevation view of a diffusion vacuum pump system according to this invention;

FIG. 2 is a partial sectional plan view of the system shown in FIG. 1;

FIG. 3 is an enlarged partial sectional view of the pumping fluid creepage barrier shown in FIG. 1; and

FIG. 4 is an enlarged partial sectional. view of another pumping fluid creepa-ge barrier embodiment of the subject invention. 1

Referring now to FIGS. 1-3 thereis shown a diffusion vacuum pump system 11 having a diffusion pump 12 con nected to a vacuum sealed high vacuum chamber 13 to be evacuated. Connecting the diffusion pump 12 and the high vacuum chamber 13 are a conventional liquid nitrogen cold trap 14 and a conventional water cooled cold trap 15. The connections are made in a vacuum tight manner so that the inner walls 16 and 17 of the liqu-id nitrogen 14 and Water cooled trap 15, respectively, form a continuous vacuum wall 20 between the diffusion pump 12 and the high vacuum chamber 13.

The lower flanged end 18 of the liquid nitrogen trap 14 is sealed to the upper flanged end 19 of water cooled cold trap 15 by an annular gasket 21. Maintaining the annular gasket 21 in position is an annular retaining ring 22 which projects into the internal area joining the liquid nitrogen trap 14 and the water cooled trap 15. Attached to the lower projecting edge 23 of the retaining ring 22 by an annular space-r 24 and a plurality of bolts 25 is the annular pumping fluid creepage barrier 26.

As shown more clearly in FIGS. 2 and 3 the annular creepage barrier 26 has a rectangular cross section with a width which is several times its height. The outer edge of the cree-page barrier 26 is attached to the retaining ring 22 While the inner portion of the barrier extends into the internal area joining the liquid nitrogen trap 14 and the Water cooled trap 15.

Thus the continuous annular creepage barrier disc 26 completely interrupts all internal surface paths along the vacuum wall 211 joining the diffusion pump 12 and high vacuum chamber 13. Surface migrating pumping fluid from the diffusion pump 12 must therefore pass overthe creepage barrier 26 before reaching high vacuum chamber 13.

The present invention utilizes for the creepage barrier 26 a material which will not be Wetted by the pumping fluid. In this way the migration or creepage of pumping fluid, along the vacuum systems internal wall can be effectively stopped.

The prefered embodiment of the invention shown in FIGS. 13 is particularly well suited for the function since since the internal projection of rectangular cross sectional creepage barrier 26 considerably lengthens the non- Wettable material path which must be traversed by the creeping pumping fluid. Also the extending spacer ring 24 may also be made of such a material to further extend the barrier length.

The positioning of the creepage barrier 26 between the condensed gas trap 14 and the water cooled trap is also of considerable advantage. Thi placement provides the barrier in an area where most of the potential barrier bypassing pumping fluid vapor has been obstructed by the water cooled trap 15 while still providing a long arduou path through condensed gas trap 14 for any vapor evaporating from the migrating surface film stopped by the creepage barrier 25.

In the operation of the preferred embodiment shown, all parts of the system above the creepagc are preheated to clear the system of pumping fluid vapors down to a partial pressure corresponding tovapor pressure of the fluid at the water temperature of the water cooled trap 15. The deep cooling of condensed gas trap 14 will then rapidly condense most remaining vapor to further reduce the pumping fluid vapor partial pressure,

FIG. 4 shows a partial view of another preferred embodiment of thi invention wherein components identical to those shown in FIG. 1 are given corresponding identical numbers. The continuous annular creepage barrier 31 shown is of L shape with one arm 32 attached to retaining ring 22 and the other arm 33 extending above the retaining ring 22. This embodiment has the advantage that projecting arm 33 together with cold surface 34 completely shields the relatively warm internal surface 16 of the condensed gas trap 14 from evaporated pumping fluid vapor.

It has been found that solid materials having a surface energy less than that exhibited by customarily used pumping fluids (such as, for example, oil) will possess the non-wetting property discussed above. Condensed fluorinated hydrocarbon materials have proven particularly effective as diffusion pump oil creepage barriers. These materials include, for example, the products sold under the trade name Teflon, Chemelec-300, Fluoro flex-T, Fluoroplast, Goring TRW, Polypenco-T and Kel-F.

Extremely effective diffusion oil creepage prevention has been obtained with barriers composed of the thermoplastic Teflon which is a registered trade name of E. I. du Pont de Nemours & Co. The chemical formula for this material is (-CF CF and it exhibits an extremely low surface energy against air of about 6 dynes per centimeter. This material also offers other good high vacuum properties such as a low vapor pressure and outgassing.

Although the particular embodiments hown are preferred, many other designs and configurations could also prove effective. For example only, the creepage barrier 2-6 could be positioned in other parts of the system, the

barrier could be constructed of other materials and then coated with a non-wettable material, etc.

It is therefore to be understood that various modifications may be resorted to by thi skilled in the art with out departing from the spirit and scope of the invention,

as hereinafter defined by the appended claims.

What is claimed is:

1'. A vacuum system apparatus comprising a diffusion vacuum pump adapted to utilize a pumping fluid, a sealed vacuum chamber adapted to be evacuated by said diffusion vacuum pump, a vacuum sealed tubulation connected positioned so, as to interrupt all paths between said diffusion vacuum pump and said sealed vacuum chamber along the internal surface of said vacuum sealed tubulation and wherein said creepage barrier is composed of a solid material not wetted by the pumping fluid utilized in said diffusion pump thereby effectively preventing surface migration of the pumping fluid between said vacuum diffusion pump and said sealed vacuum chamber.

2. The apparatus according to claim 1 wherein said creepage barrier is composed of a material having a surface energy in air less than that of the pumping fluid.

3. The apparatus according to claim 1 wherein said creepage barrier is composed of a material which has a surface energy in air of less than 25 dynes per centimeter.

4. The apparatus according to claim 1 wherein said creepage barrier is composed of a compressed fluorinatcd hydrocarbon material.

5. The apparatus according to claim 1 wherein said creepage barrier is composed of the thermoplastic material sold under the trade name Teflon.

6. The apparatus according to claim 5 wherein said vaccum sealed tubulation includes a trap adapted for use with compressed cooling gas positioned adjacent said sealed vacuum chamber and a trap adapted for use with cooling water positioned adjacent said vacuum diffusion pump, and said creepage barrier is an annular ring in contact with the internal surface of said vacuum sealed tubulation and positioned between said traps.

7. A vacuum system apparatus comprising a diffusion vacuum pump adapted to utilize a pumping fluid, a sealed vacuum chamber adapted to be evacuated by said diffusion vacuum pump, a vacuum sealed tubulation connected between said diffusion vacuum pump and said sealed vacuum chamber so as to provide for gas communication therebetween, a creepage barrier including a three dimensional portion which extends into the volume enclosed by said vacuum sealed tubulation and thereby serves to shield said vacuum chamber from the pumping fluid which evaporates in said diffusion pump, said creepage barrier positioned so as to interrupt all paths between said diffusion vacuum pump and said sealed vacuum chamber v along the internal surface of said vacuum sealed tubulation, and wherein said creepage barrier is composed of the thermoplastic material sold under the trade name Teflon thereby effectively preventing migration of the pumping fluid between said diffusion vacuum pump and said sealed vacuum chamber.

8. The apparatus according to claim 7 wherein said vacuum sealed tubulation includes a trap adapted for use with compressed cooling gas positioned adjacent said sealed vacuum chamber and a trap adapted for use with cooling water positioned adjacent said vacuum diffusion pump, and said creepage barrier comprises an annular ring in contact with the internal surface of said vacuum sealed tubulation and positioned between said traps.

9. A vacuum system apparatus comprising a diffusion vacuum pump adapted to utilize a pumping fluid, a sealed vacuum chamber adapted to be evacuated by said diffusion vacuum pump, a vacuum sealed tubulation connected between said diffusion vacuum pump and said sealed vacuum chamber so as to provide for gas communication therebetween, a creepage barrier including a three dimensional portion which extends into the volume enclosed .by said vacuum sealed tubulation and thereby serves to shield said vacuumchamber from the pumping fluid which evaporates in said diffusion pump, said creepage barrier positioned so as to interrupt all paths between said diffusion vacuum pump and said sealed vacuum chamber along the internal surface of said vacuum sealed tubulation and wherein said creepage barrier is composed of a compressed fluoronated hydrocarbon material so as to effectively prevent migration of the pumping fluid between said vacuum diffusion pump and said sealed vacuum chamber.

lit. The apparatus according to claim 9 wherein said vacuum sealed tubulation includes a trap adapted for use with compressed cooling gas positioned adjacent said sealed vacuum chamber and a trap adapted for use with cooling water positioned adjacent said vacuum diffusion pump, and said creepage barrier comprises an annular ring in contact with the internal surface of said vacuum sealed tubulation and positioned between said traps.

11. A vacuum system apparatus comprising a diffusion vacuum pump adapted to utilize a pumping fluid, a sealed vacuum chamber adapted to be evacuated by said diffusion vacuum pump, a vacuum sealed tubulation connected between said diffusion vacuum pump and said sealed vacuum chamber so as to provide for gas communication therebetween, a creepage barrier including a three dimensional portion which extends into the volume enclosed by said vacuum sealed tubulation and thereby serves to shield said vacuum chamber from the pumping fluid which evaporates in said diffusion pump, said creepage barrier positioned so as to interrupt all paths between said diffusion vacuum pump and said sealed vacuum chamber along the internal surface of said vacuum sealed tubulation and wherein said creepage barrier is composed of a material having a surface energy of less than 25 dynes per centimeter so as to effectively prevent migration of the pumping fluid between said vacuum diffusion pump and said sealed vacuum chamber.

-12. The apparatus according to claim 11 wherein said vacuum sealed tubulation includes a trap adapted for use with compressed cooling gas positioned adjacent said sealed vacuum chamber and a trap adapted for use with cooling water positioned adjacent said vacuum diffusion pump, and said creepage barrier comprises an annular ring in contact with the internal surface of said vacuum sealed tubulation and positioned between said traps.

13. A vacuum system apparatus comprising a diffusion vacuum pump adapted to utilize a pumping fluid, a sealed vacuum chamber adapted to be evacuated by said diffusion vacuum pump, a vacuum sealed tubulation connected between said diffusion vacuum pump and said sealed vacuum chamber so as to provide for gas communication therebetween, a creepage barrier including a three dimensional portion which extends into the volume enclosed by said vacuum sealed tubulation and thereby serves to shield said vacuum chamber from the pumping fluid which evaporates in said diffusion pump, said creepage barrier positioned so as to interrupt all paths between said diffusion vacuum pump and said sealed vacuum chamber along the internal surface of said vacuum sealed tubulation, and wherein said creepage barrier is composed of a material having a surface energy in air less than that of the pumping fluid so as to effectively prevent migration of the pumping fluid between said diffusion vacuum pump and said sealed vacuum chamber.

14. The apparatus according to claim 13 wherein said vacuum sealed tubulation includes a trap adapted for use with compressed cooling gas positioned adjacent said sealed vacuum chamber and a trap adapted for use with cooling water positioned adjacent said vacuum diffusion pump, and said creepage barrier comprises an annular ring in contact with the internal surface of said vacuum sealed tubulation and positioned between said traps.

'15. A device for use in diffusion vacuum pump systems including a diffusion vacuum pump adapted for use with a pumping fluid, a vacuum chamber to be evacuated, and a vacuum sealed tubulation providing for gas communication therebetween, said device comprising a creepage barrier adapted for placement in the diffusion vacuum pump system in such a manner as to interrupt all paths between the diffusion vacuum pump and the vacuum chamber along the internal surface of the vacuum sealed tubulation, said creepage barrier comprising a three dimensional portion adapted to extend into the volume enclosed by the vacuum sealed tubulation and wherein said creepage barrier is composed of a material not wetted by the pumping fluid.

16. A device for use in diffusion vacuum pump systems including a diffusion vacuum pump adapted for use with a pumping fluid, a vacuum chamber to be evacuated, and a vacuum sealed tubulation providing for gas communication therebetween, said device comprising a creepage barrier adapted for placement in the diffusion vacuum pump system in such a manner as to interrupt all paths between the diffusion vacuum pump and the vacuum chamber along the internal surface of the vacuum sealed tubulation, said creepage barrier comprising a three d-imensional portion adapted to extend into the volume enclosed by the vacuum sealed tubulation and wherein said creepage barrier is composed of a material having a surface energy in air less than that of the pumping fluid.

17. A device for use in diffusion vacuum pump systems including a diffusion vacuum pump adapted for use with a pumping fluid, a vacuum chamber to be evacuated, and a vacuum sealed tubulation providing for gas communication therebetween, said device comprising a creepage barrier adapted for placement in the diffusion vacuum pump system in such a manner as to interrupt all paths between the diffusion vacuum pump and the vacuum chamber along the internal surfaceof the vacuum sealed tubulation, said creepage barrier comprising a three dimensional portion adapted to extend into the volume enclosed by the vacuum sealed tubulation and wherein said creepage barrier is composed of a material which has a surface energy in air of less than 25 dynes per centimeter.

18. A device for use in diffusion vacuum pump systems including a diffusion vacuum pump adapted for use with a pumping fluid, a vacuum chamber to be evacuated, and a Vacuum sealed tubulation providing for gas communication therebetween, said device comprising a creepage barrier adapted for placement in the diffusion vacuum pump system in such a manner as to interrupt all paths between the diffusion vacuum pump and the vacuum chamber along the internal surface of the vacuum sealed tubulation, said creepage barrier comprising a three dimensional portion adapted to extend int-o the volume enclosed by the vacuum sealed tubulation and wherein said creepage barrier is made of a compressed fluorinated hydrocarbon material.

19. A device for use in diffusion vacuum pump systems including a diffusion vacuum pump adapted for use with a pumping fluid, a vacuum chamber to be evacuated, and a vacuum sealed tubulation providing for gas communication therebetween, said device comprising a creepage barrier adapted for placement in the diffusion vacuum pump system in such a manner as to interrupt all paths between the diffusion vacuum pump and the vacuum chamber along the internal surface of the-vacuum sealed tubulation, said creepage barrier comprising a three dimensional portion adapted to extend into the volume enclosed by the vacuum sealed tubulation and wherein said creepage barrier is composed of the thermoplastic material sold under the trade name Teflon,

20. A vacuum system apparatus comprising a diffusion vacuum pump adapted to utilize a pumping fluid, a sealed vacuum chamber adapted to be evacuated by said diffusion vacuum pump, a vacuum sealed tubulation connected between said diffusion vacuum pump and said sealed vacuum chamber so as to provide for gas communication therebetween, a creepage barrier located entirely within said vacuum sealed tubulation so as to be isolated from the atmosphere during periods in which said tubulation is maintained at reduced pressure, said creepage barrier including a three dimensional portion which extends into the volume enclosed by said vacuum sealed tubulation and positioned so as to interrupt all paths between said diffusion vacuum p ump and said sealed vacuum chamber along the internal surface of said vacuum sealed tubulation, and wherein said creepage barrier is composed of the thermoplastic material sold under the trade name Teflon thereby preventing surface migration of the pumping fluid between said diffusion vacuum pump and said sealed vacuum chamber.

uum chamber so as to provide for gas communication therebetween, a creepage barrier located within said vacuum sea-led tubulation, said creepage barrier comprising a three dimensional portion positioned in the volume enclosed by said vacuum sealed tubulati-on thereby serving to shield said vacuum chamber from the pumping fluid which evaporates in said diffusion vacuum pump, and wherein said three dimensional portion is composed of a solid material not wetted by the pumping fluid utilized in said diflusion pump thereby effectively preventing surface migration of the pumping fluid in a direction towards said sealed vacuum chamber.

22. The apparatus according to claim 21 wherein said three dimensional portion is composed of a material having a surface energy in air less than that of the pumping fluid.

23. The apparatus according to claim 21 wherein said three dimensional portion is composed of a material which has a surface energy in air of less than 25 dynes per centimeter.

24. The apparatus according to claim 21 wherein said three dimensional portion is composed of a compressed fluorina-ted hydrocarbon material.

25. The apparatus according to claim 21 wherein said three dimensional portion is composed of the thermoplastic material sold under the trade name Teflon thereby preventing surface migration of the pumping fluid between said diffusion vacuum pump and said sealed vacuum chamber.

References Cited by the Examiner UNETED STATES PATENTS 3,077,638 2/63 Hickam 285-363 XR FOREIGN PATENTS 1,202,397 7/59 France. 1,254,532 1/61 France.

M. CARY NELSON, Primary Examiner. 

1. A VACUUM SYSTEM APPARATUS COMPRISING A DIFFUSION VACUUM PUMP ADAPTED TO UTILIZE A PUMPING FLUID, A SEALED VACUUM CHAMBER ADAPTED TO BE EVACUATED BY SAID DIFFUSION VACUUM PUMP, A VACUUM SEALED TUBULATION CONNECTION BETWEEN SAID DIFFUSION VACUUM PUMP AND SAID SEALED VACUUM CHAMBER SO AS TO PROVIDE FOR GAS COMMUNICATION THEREBETWEEN, A CREEPAGE BARRIER LOCATED ENTIRELY WITHIN SAID VACUUM SEALED TUBULATION SO AS TO BE ISOLATED FROM THE ATMOSPHERE DURING PERIODS IN WHICH SAID TUBULATION IS MAINTAINED AT REDUCED PRESSURE, SAID CREEPAGE BARRIER POSITIONED SO AS TO INTERRUPT ALL PATHS BETWEEN SAID DIFFUSION VACCUUM PUMP AND SAID SEALED VACUUM CHAMBER ALONG THE INTERNAL SURFACE OF SAID VACUUM SEALED TUBULATION AND WHEREIN SAID CREEPAGE BARRIER IS COMPOSED OF A SOLID MATERIAL NOT WETTED BY THE PUMPING FLUID UTILIZED IN SAID DIFFUSION PUMP THEREBY EFFECTIVELY PREVENTING SURFACE MIGRATION OF THE PUMPING FLUID BETWEEN SAID VACUUM DIFFUSION PUMP AND SAID SEALED VACUUM CHAMBER. 